Drum washing machine with hinge units

ABSTRACT

Disclosed herein is a drum washing machine. A rotational track of a first door to open and close an opening of a machine body is concentric with a rotational track of a second door to open and close an opening of a water tub, thereby improving user convenience.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Korean Patent Application No. 2009-0114545, filed on Nov. 25, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Field

Embodiments relate to a drum washing machine using two doors to reduce vibration.

2. Description of the Related Art

Generally, a drum washing machine washes laundry through tumbling of the laundry in a rotary tub during rotation of the rotary tub. The drum washing machine includes a water tub elastically supported in a machine body, a rotary tub rotatably mounted in the tub, a motor mounted at the rear of the water tub to drive the rotary tub, a water supply device to supply water, and a drainage device to drain water.

At the front of the machine body is provided a laundry introduction port through which laundry is introduced into the rotary tub. At the fronts of the water tub and the rotary tub are provided openings corresponding to the laundry introduction port.

Also, the laundry introduction port of the machine body is connected to the opening of the water tub via a diaphragm, and a door is mounted at the laundry introduction port, to prevent leakage of wash water during washing and spin-drying cycles.

The diaphragm exhibits nonlinearity and large deformation properties. When the displacement of the water tub is great in transition duration, vibration generated from the water tub is transmitted to the machine body via the diaphragm, with the result that the machine body is vibrated.

In recent years, a washing machine has been developed in which the laundry introduction port of the machine body is not connected to the opening of the water tub via the diaphragm, but, instead, an additional door is mounted at the opening of the water tub, to prevent transmission of vibration to the machine body via the diaphragm.

SUMMARY

It is an aspect of the at least one embodiment to provide a drum washing machine in which the transmission of vibration from a water tub to a machine body is reduced.

It is another aspect of the at least one embodiment to provide a drum washing machine in which the size of an inside door is increased to improve user convenience.

Additional aspects will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

The foregoing and/or other aspects are achieved by providing a drum washing machine including a machine body having a first opening, a water tub disposed in the machine body, the tub having a second opening facing the first opening, a rotary drum rotatably provided in the tub to contain laundry, a first door to open and close the first opening, a second door to open and close the second opening, a first hinge unit mounted at the machine body to rotatably support the first door, and a second hinge unit mounted to the tub to rotatably support the second door such that a rotational center of a rotation track of the second door coincides with a rotational center of a rotation track of the first door during opening of the second door.

The drum washing machine may further include a coupling unit to couple the first door and the second door such that the second door is simultaneously opened when the first door is opened.

The coupling unit may be provided at the first door, and the coupling unit may include a hook unit configured to be hooked by and released from a hook coupling part provided at the second door.

The coupling unit may further include an actuator to release the hook unit.

The actuator may include a drive motor, a wire coupled to the drive motor, and an idle gear configured to be rotated by movement of the wire, and the hook unit may be rotated about a hook rotation shaft. Also, hook unit may be provided at one end thereof with a gear engaged with the idle gear and at the other end thereof with a hook coupled to the second door.

The actuator may release the hook unit during an operation of the drum washing machine in a state in which the first opening and the second opening are closed.

The coupling unit may further include a hook holding lever to hold the hook unit released by the actuator to maintain a released state of the hook unit.

The second hinge unit may have a rotation center located on the same axis as the rotation center of the first door.

The first hinge unit may include first hinge arms provided at the machine body such that the first hinge arms are vertically spaced apart from each other, each of the first hinge arms having a first rotation shaft hole formed at an end thereof, and second hinge arms provided at the first door such that the second hinge arms are vertically spaced apart from each other, each of the second hinge arms having a second rotation shaft hole formed at an end thereof such that the second rotation shaft hole is aligned with the corresponding first rotation shaft hole. The first hinge unit may further include a first hinge shaft coupling the first hinge arms and the second hinge arms at the first and second rotation shaft holes. Also, the second hinge unit may include third hinge arms provided at the tub such that the third hinge arms are vertically spaced apart from each other, the third hinge arms extending from the tub toward a front of the machine body such that the third hinge arms reach positions corresponding to the first rotation shaft holes, each of the third hinge arms having a third rotation shaft hole formed at an end thereof, and fourth hinge arms provided at the second door such that the fourth hinge arms are vertically spaced apart from each other, the fourth hinge arms being bent from the second door and extending toward the front of the machine body, each of the fourth hinge arms having a fourth rotation shaft hole formed at an end thereof such that the fourth rotation shaft hole is aligned with the corresponding third rotation shaft hole. The second hinge unit may further include a second hinge shaft coupling the third hinge arms and the fourth hinge arms at the third and fourth rotation shaft holes

The first door may be rotated about a first rotation shaft of the first hinge unit, the second door may be rotated about a second rotation shaft of the second hinge unit, and the second hinge unit may include a second hinge arm curvedly protruding from one side of the second door in a lateral direction thereof, the second hinge arm having a curved guide groove in which the second rotation shaft is disposed such that the second rotation shaft is guided along the guide groove.

The guide groove may have a regular radius of rotation about the first rotation shaft.

The second hinge unit may include a hinge bracket fixed to the tub, the hinge bracket having a first link shaft, a first link plate having one end rotatably coupled to the first link shaft and the other end coupled to a second link shaft, a second link plate rotatably coupled to the second link shaft, the second link plate being curved toward the first link shaft, the second link plate having a second hinge shaft provided at the end thereof, and a second hinge arm curvedly protruding from one side of the second door in a lateral direction thereof such that the second hinge arm is guided along support planes provided at a top and bottom of the second link plate, the second hinge arm having a curved guide groove in which the second hinge shaft is disposed.

The first link shaft may be provided with a first torsion spring to elastically push the first link plate in a direction opposite to a direction in which the first link plate is rotated, and the second link shaft may be provided with a first torsion spring to elastically push the second link plate in a direction identical to a direction in which the second link plate is rotated.

The foregoing and/or other aspects are achieved by providing a drum washing machine including a machine body having a first opening, a tub disposed in the machine body, the tub having a second opening facing the first opening, a drum rotatably provided in the tub to contain laundry, a first door rotatably provided at the machine body to open and close the first opening, and a second door rotatably provided at the tub to open and close the second opening, wherein a distance between a first rotational track formed by an end of the first door and a second rotational track formed by an end of the second door is uniformly maintained upon opening and closing of the first door and the second door.

The first door may be provided with a coupling unit to couple the first door and the second door such that the second door is simultaneously opened when the first door is opened.

The coupling unit may include a hook unit configured to be hooked by and released from a hook coupling part provided at the second door.

The coupling unit may further include an actuator to actuate the hook unit to be released from the hook coupling part and a hook holding lever to hold the hook unit released by the actuator.

The actuator may release the hook unit during an operation of the drum washing machine.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a perspective view illustrating a drum washing machine according to at least one embodiment with doors separated from the drum washing machine;

FIG. 2 is a sectional view of the drum washing machine of FIG. 1;

FIG. 3 is a view illustrating operations of the doors of the drum washing machine of FIG. 1;

FIG. 4 is an exploded perspective view illustrating a door hinge structure according to at least one embodiment;

FIG. 5 is a view illustrating the internal structure of a coupling unit according to at least one embodiment;

FIG. 6 is a sectional view illustrating a door hinge structure according to at least one embodiment; and

FIGS. 7 to 10 are views illustrating a door hinge structure according to at least one embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to the at least one embodiment, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

Referring to FIGS. 1 and 2, a drum washing machine according to at least one embodiment includes a machine body 10 forming the external appearance thereof, a water tub 20 mounted in the machine body 10 to contain wash water, and a rotary tub (drum) 14 rotatably mounted in the water tub 20, the rotary tub 14 having a plurality of spin-drying holes formed therethrough.

A control panel 15 to control the drum washing machine may be provided at the upper front of the machine body 10, although is not limited to such placement. A detergent box 16 may be mounted at one side of the control panel 15, although is not limited to such placement.

In the upper part of the machine body may be provided a water supply device 17 to supply wash water to the detergent box 16 and the water tub 20. In the lower part of the machine body 10 may be provided a drainage device 18 to drain wash water from the water tub 20 to the outside of the machine body 10.

At the rear of the water tub 20 may be provided a drive unit 19 to rotate the rotary tub 14 such that washing, rinsing, and spin-drying cycles are performed in the rotary tub 14.

At a front 13 of the machine body 10 may be provided a first opening 11 through which laundry is introduced and removed. The water tub 20 may be disposed horizontally to the machine body 10. At the front of the water tub 20 may be provided a second opening 21 facing the first opening 11 of the machine body 10.

At one side of the machine body 10 is provided a first door 30 to open and close the first opening 11. To one side of the first door 30 may be coupled a first hinge unit 40 mounted at the machine body 10.

The first hinge unit 40 (see FIG. 3) may rotatably support the first door 30 such that the first door 30 rotates about a rotational center 35 of the first hinge unit 40 in the frontward and rearward directions of the front 13 of the machine body 10 to open and close the first opening 11 of the machine body 10.

The outer circumference of the first door 30 tightly contacts the edge of the first opening 11 to maintain a hermetically-sealed state when the first opening 11 is closed by the first door 30. At the center of the first door 30 may be provided a first window 31 through which a user may look into the rotary tub 14.

At one side of the water tub 20 may be provided a second door 50 to open and close the second opening 21. To one side of the second door 50 may be coupled a second hinge unit 60 mounted at the water tub 20.

At the outer circumference of the second door 50 may be provided a gasket to maintain a hermetically-sealed state when the second opening 21 is closed by the second door 50. At the center of the second door 50 may be provided a second window 51 through which a user may look into the rotary tub 14.

As shown in FIG. 3, the second hinge unit 60 may rotatably support the second door 50 such that a second rotational track 53 of the second door 50 upon opening of the second door 50 is concentric to a first rotational track 33 of the first door 30 upon rotation of the first door 30 about the rotational center 35 of the first hinge unit 40.

That is, when the first door 30 and the second door 50 are opened, a distance D between the first rotational track 33 formed by the end of the first door 30 and the second rotational track 53 formed by the end of the second door 50 may be uniform.

In this structure, a center C1 of the first door 30 may coincide with a center C2 of the second door 50 when the first door 30 and the second door 50 are simultaneously opened.

As a result, interference between the second door 50 and the first opening 11 when the second door 50 is opened may be reduced. Consequently, the size of the second door 50 may be increased, and therefore, the introduction of laundry through the second opening 21 may be easily achieved.

The second hinge unit 60 may be formed in various shapes as long as the second hinge unit 60 rotatably supports the second door 50 such that the rotational center 35 of the second rotational track 53 of the second door 50 coincides with the rotational center 35 of the first rotational track 33 of the first door 30.

An embodiment of the second hinge unit 60 will be described with reference to FIG. 4.

Referring to FIG. 4, the first hinge unit 40 to rotatably support the first door 30 may include a first hinge arm 41 fixed to the machine body 10, the first hinge arm 41 having a first rotation shaft hole 43, and a second hinge arm 44 fixed to the first door 30, the second hinge arm 44 having a second rotation shaft hole 45 corresponding to the first rotation shaft hole 43.

The first hinge arm 41 curvedly protrudes from the machine body 10 toward the water tub 20. The first rotation shaft hole 43 may be formed at the end of the first hinge arm 41. The second hinge arm 44 may include a first arm portion 46 protruding rearward from the first door 30, a second arm portion 47 bent from the end of the first arm portion 46 and extending in the horizontal direction, and a third arm portion 48 bent from the end of the second arm portion 47 and extending toward the first door 30. The second rotation shaft hole 45 is formed at the end of the third arm portion 48. Consequently, the second hinge arm 44 may be formed approximately in the shape of “[”.

Two first hinge arms 41 may be disposed at the upper and lower sides of a first hinge bracket 49 mounted at the machine body 10 such that the first hinge arms 41 are vertically spaced apart from each other. Two second hinge arms 44 may be disposed at the upper and lower sides of the first door 30 such that the second hinge arms 44 are vertically spaced apart from each other.

Also, when the first door 30 is mounted at the machine body 10, the first rotation shaft hole 43 of the first hinge arm 41 and the second rotation shaft hole 45 of the second hinge arm 44 are arranged adjacent to each other, and a first hinge shaft 42 is inserted through the first rotation shaft hole 43 and the second rotation shaft hole 45.

The second hinge unit 60 to rotatably support the second door 50 may include a third hinge arm 61 fixed to the water tub 20, the third hinge arm 61 having a third rotation shaft hole 62, and a fourth hinge arm 64 fixed to the second door 50, the fourth hinge arm 64 having a fourth rotation shaft hole 65.

The third hinge arm 61 protrudes from the water tub 20 toward the front of the machine body 10. The third rotation shaft hole 62 may be formed at the end of the third hinge arm 61. The third hinge arm 61 may extend to a position corresponding to the first rotation shaft hole 43 of the first hinge arm 41 fixed to the machine body 10 such that the first rotation shaft hole 43 and the third rotation shaft hole 62 have the same rotational axis 68. Consequently, the rotational center of the first door 30 and the rotational center of the second door 50 are located on the same axis.

The fourth hinge arm 64 may include a first arm portion 66 extending laterally from one side of the second door 50 and a second arm portion 67 bent from the end of the first arm portion 66 and extending toward the front of the machine body 10. The fourth rotation shaft hole 65 may be formed at the end of the second arm portion 67.

The third hinge arm 61 and the fourth hinge arm 64 may be coupled to each other by a second hinge shaft 63 inserted through the third rotation shaft hole 62 and the fourth rotation shaft hole 65 to rotatably support the second door 50 at one side of the water tub 20.

In this structure, the rotational center of the first hinge unit 40 and the rotational center of the second hinge unit 60 coincide with each other. Therefore, when the first door 30 and the second door 50 are opened, as shown in FIG. 3, the rotational tracks formed by the ends of the first door 30 and the second door 50 are concentric, with the result that relative movement between the first door 30 and the second door 50 does not occur during rotation of the first door 30 and the second door 50.

Consequently, relative sliding movement between the first door 30 and the second door 50 does not occur during rotation of the first door 30 and the second door 50, and therefore, the size of the second door 50 may be increased such that laundry is easily introduced and removed.

Referring to FIGS. 1 and 3, a coupling unit 70 to couple the first door 30 and the second door 50 such that the second door 50 is simultaneously opened upon opening of the first door 30 may be provided at the first door 30.

The coupling unit 70 may include a first hook unit 71 configured to be coupled to and released from the second door 50. The first hook unit 71 may be coupled to a first hook coupling part 76 provided at the second door 50.

When the first door 30 and the second door 50 are coupled to each other by the coupling unit 70, the first door 30 and the second door 50 may be simultaneously rotated. Consequently, a user may move only one of the doors, thereby improving user convenience.

Referring to FIG. 5, the coupling unit 70 may include an actuator 80 to release the first hook unit 71 and a hook holding lever 90 to hold the first hook unit 71 released by the actuator 80.

The first hook unit 71 may be rotated about a hook rotation shaft 72. The first hook unit 71 may be provided at one end thereof with a gear 73. The first hook unit 71 may be provided at the other end thereof with a hook 74 hooked by the first hook coupling part 76 provided at the second door 50.

The actuator 80 rotates the hook 74 about the hook rotation shaft 72 such that the hook 74 is released from the first hook coupling part 76 provided at the second door 50.

The actuator 80 may include a drive motor 81, a wire 83, for example, coupled to the drive motor 81 such that the wire 83 is pulled by the rotation of the drive motor 81, and an idle gear 85 coupled to the wire 83 such that the idle gear 85 is engaged with the gear 73 of the first hook unit 71. Also, a restoring spring 75 to restore the first hook unit 71 rotated by the actuator 80 to the original position thereof may be provided on the hook rotation shaft 72.

Consequently, when the drive motor 81 is rotated to pull the wire 83, the wire 83 rotates the idle gear 85. As a result, the gear 73 of the first hook unit 71 engaged with the idle gear 85 is rotated, and therefore, the hook 74 of the first hook unit 71 is released from the first hook coupling part 76 provided at the second door 50.

The hook holding lever 90 may hold the first hook unit 71 released from the first hook coupling part 76 to maintain a released state of the first hook unit 71.

The hook holding lever 90 may be rotated about a lever rotation shaft 91. At one end of the hook holding lever 90 may be provided a lever part 93 protruding to the side of the first door 30. At the other end of the hook holding lever 90 may be provided a catching part 95 to support the first hook unit 71. Also, a lever spring 97 to elastically push the hook holding lever 90 toward the first hook unit 71 may be provided on the lever rotation shaft 91.

Consequently, the first hook unit 71 released from the first hook coupling part 76 by the actuator 80 is supported by the hook holding lever 90, thereby preventing consumption of power used for the actuator 80 to maintain the released state of the first hook unit 71.

During the operation of the drum washing machine in a state in which the first door 30 and the second door 50 are closed, a controller (not shown) controls the first hook unit 71 of the coupling unit 70 to be rotated such that the coupling between the first door 30 and the second door 50 is released.

During the operation of the drum washing machine, therefore, vibration from the water tub 20 is not transmitted to the machine body 10 via the first door 30.

When the first door 30 is opened, on the other hand, a user may rotate the lever part 93 of the hook holding lever 90 protruding to the side of the first door 30, with the result that the first hook unit 71 is released from the catching part 95 and hooked by the first hook coupling part 76 of the second door 50.

Subsequently, when the user pulls the first door 30, the pulling force is transmitted to the second door 50 by the first hook unit 71, and therefore, the second door 50 is pulled, with the result that the first door 30 and the second door 50 are simultaneously opened.

FIG. 6 is a sectional view illustrating a door hinge structure according to at least one embodiment. Components of this embodiment identical to those of the previous embodiment are denoted by the same reference numerals, and a detailed description thereof is omitted.

Referring to FIG. 6, a first door 30 may be rotated about a first rotation shaft 111 of a first hinge unit 110 mounted at a machine body 10, and a second door 50 may be rotated about a second rotation shaft 114 of a second hinge unit 113 mounted at a water tub 20.

In this case, the second hinge unit 113 may include a second hinge arm 116 curvedly protruding from one side of the second door 50 in the lateral direction thereof such that a second rotational track 115 of the second door 50 is concentric with a first rotational track 112 of the first door 30.

At the second hinge arm 116 may be provided a curved guide groove 117 in which the second rotation shaft 114 is disposed such that the second rotation shaft 114 is guided along the guide groove 117. The guide groove 117 may have a regular radius of rotation R about the first rotation shaft 111 when the second door 50 is rotated.

Consequently, when the second door 50 is rotated open along with the first door 30, a uniform distance D is maintained between the second rotational track 115 of the second door 50 and the first rotational track 115 of the first door 30 by the second hinge unit 113, and therefore, the first door 30 and the second door 50 are simultaneously rotated without relative sliding movement between the first door 30 and the second door 50.

FIGS. 7 to 10 are views illustrating a door hinge structure according to at least one embodiment. Components of this embodiment identical to those of the previous embodiments are denoted by the same reference numerals, and a detailed description thereof is omitted.

In this embodiment, a second hinge unit 130 may further include a link structure to enable a second door 50 having a curved second hinge arm 131 to be easily opened and closed.

Referring to FIG. 7, a first door 30 may be rotated about a first rotation shaft 121 of a first hinge unit 120 mounted at a machine body 10 to open and close a first opening 11, and the second door 50 may be rotated by a second hinge unit 130 mounted at a water tub 20 to open and close a second opening 21.

Referring to FIGS. 8 and 9, the second hinge unit 130 may include a hinge bracket 133 fixed to the water tub 20, the hinge bracket 133 having a first link shaft 135, a first L-shaped link plate 140 having one end coupled to the first link shaft 135 and the other end coupled to a second link shaft 141, and a second link plate 150 rotatably coupled to the portions of the second link shaft 141 protruding upward and downward from the first link plate 140, the second link plate 150 being curved toward the first link shaft 135 at a predetermined curvature, the second link plate 150 having a second hinge shaft 151 provided at the end thereof.

The first link plate 140 may be rotated counterclockwise about the first link shaft 135. On the first link shaft 135 may be provided a first torsion spring 136 to elastically push the first link plate 140 in the clockwise direction, for example.

The second link plate 150 may be rotated clockwise about the second link shaft 141. On the second link shaft 141 may be provided a second torsion spring 143 to elastically push the second link plate 150 in the clockwise direction, for example. However, the embodiment is not limited to the torsion springs 136, 143 and may include any type of devices that pushes the first and second link plates 140, 150 in one direction.

The second link plate 150 may be provided at the top and bottom thereof with support planes 153 to support second hinge arms 131 curvedly protruding from one side of the second door 50 in the lateral direction thereof. At opposite sides of the support planes 153 may be provided ribs 155 to prevent the second hinge arms 131 located on the support planes 153 from being separated from the support planes 153.

Consequently, when the first door 30 and the second door 50 are simultaneously opened, as shown in FIG. 10, the first link plate 140 is rotated counterclockwise about the first link shaft 135, and the second link plate 150 is rotated clockwise about the second link shaft 141. Also, the second hinge arms 131 are moved while guide grooves 132 of the second hinge arms 131 are guided by the second hinge shaft 151 in the rotation direction. As a result, the second hinge unit 130 may rotate the second door 50 at a regular radius of rotation.

When the second door 50 is opened, the second torsion spring 143 elastically pushes the second link plate 150 in the opening direction of the second door 50, and therefore, the second door 50 is easily opened.

Meanwhile, the first door 30 and the second door 50 are simultaneously rotated without sliding between the first door 30 and the second door 50 upon opening of the first door 30 and the second door 50 through the structure of the embodiment of FIG. 4 in which the rotational center of the second door and the rotational center of the first door are located on the same axis, through the structure of the embodiment of FIG. 6 in which the second hinge unit has the curved hinge arm, or through the structure of the embodiment of FIGS. 7 to 10 in which the second hinge unit has the link structure. However, the second hinge unit is not limited thereto and may be formed in any shape as long as the second hinge unit prevents relative movement between the first door 30 and the second door 50 upon simultaneous opening of the first door 30 and the second door 50.

As is apparent from the above description, vibration generated from the water tub of the drum washing machine is prevented from being transmitted to the machine body.

Also, the size of the door to open and close the opening of the water tub of the drum washing machine may be increased, thereby improving user convenience.

Also, the two doors of the drum washing machine are simultaneously rotated, thereby providing an aesthetically pleasing appearance.

Although at least one embodiment has been shown and described, it would be appreciated by those skilled in the art that changes may be made in the at least one embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents. 

1. A drum washing machine, comprising: a machine body having a first opening; a water tub disposed in the machine body, the tub having a second opening facing the first opening; a rotary drum rotatably provided in the water tub to contain laundry; a first door to open and close the first opening; a second door to open and close the second opening; a first hinge unit mounted at the machine body to rotatably support the first door; and a second hinge unit mounted to the water tub to rotatably support the second door such that a rotational center of a rotational track of the second door coincides with a rotational center of a rotational track of the first door during opening of the second door.
 2. The drum washing machine according to claim 1, further comprising a coupling unit to couple the first door and the second door such that the second door is simultaneously opened when the first door is opened.
 3. The drum washing machine according to claim 2, further comprising a hook coupling part provided at the second door, and wherein the coupling unit is provided at the first door, and the coupling unit includes a hook unit configured to be hooked by and released from the hook coupling part provided at the second door.
 4. The drum washing machine according to claim 3, wherein the coupling unit further includes an actuator to release the hook unit from the hook coupling part.
 5. The drum washing machine according to claim 4, wherein the actuator includes a drive motor, a wire coupled to the drive motor, and an idle gear configured to be rotated by movement of the wire, and the hook unit is rotated about a hook rotation shaft, the hook unit being provided at one end thereof with a hook gear engaged with the idle gear and at the other end thereof with a hook configured to be coupled to hook coupling part of the second door.
 6. The drum washing machine according to claim 4, wherein the actuator releases the hook unit during an operation of the drum washing machine in a state in which the first opening and the second opening are closed by the first and second doors.
 7. The drum washing machine according to claim 6, wherein the coupling unit further includes a hook holding lever to hold the hook unit released by the actuator to maintain a released state of the hook unit.
 8. The drum washing machine according to claim 1, wherein the second hinge unit has a rotational center located on the same axis as the rotational center of the first door.
 9. The drum washing machine according to claim 8, wherein: the first hinge unit includes first hinge arms provided at the machine body such that the first hinge arms are vertically spaced apart from each other, each of the first hinge arms having a first rotation shaft hole formed at an end thereof, and second hinge arms provided at the first door such that the second hinge arms are vertically spaced apart from each other, each of the second hinge arms having a second rotation shaft hole formed at an end thereof such that the second rotation shaft hole is aligned with the corresponding first rotation shaft hole, the first hinge unit further including a first hinge shaft coupling the first hinge arms and the second hinge arms at the first and second rotation shaft holes, and the second hinge unit includes third hinge arms provided at the water tub such that the third hinge arms are vertically spaced apart from each other, the third hinge arms extending from the water tub toward a front of the machine body such that the third hinge arms reach positions corresponding to the first rotation shaft holes, each of the third hinge arms having a third rotation shaft hole formed at an end thereof, and fourth hinge arms provided at the second door such that the fourth hinge arms are vertically spaced apart from each other, the fourth hinge arms being bent from the second door and extending toward the front of the machine body, each of the fourth hinge arms having a fourth rotation shaft hole formed at an end thereof such that the fourth rotation shaft hole is aligned with the corresponding third rotation shaft hole, the second hinge unit further including a second hinge shaft coupling the third hinge arms and the fourth hinge arms at the third and fourth rotation shaft holes.
 10. The drum washing machine according to claim 1, wherein the first door is rotated about a first rotation shaft of the first hinge unit, the second door is rotated about a second rotation shaft of the second hinge unit, and the second hinge unit includes a hinge arm curvedly protruding from one side of the second door in a lateral direction thereof, the hinge arm having a curved guide groove in which the second rotation shaft is disposed such that the second rotation shaft is guided along the curved guide groove.
 11. The drum washing machine according to claim 10, wherein the curved guide groove has a regular radius of rotation about the first rotation shaft.
 12. The drum washing machine according to claim 1, wherein the second hinge unit comprises: a hinge bracket fixed to the water tub, the hinge bracket having a first link shaft; a first link plate having one end rotatably coupled to the first link shaft and the other end coupled to a second link shaft; a second link plate rotatably coupled to the second link shaft, the second link plate being curved toward the first link shaft, the second link plate having a hinge shaft provided at the end thereof; and a hinge arm curvedly protruding from one side of the second door in a lateral direction thereof such that the hinge arm is guided along support planes provided at a top and a bottom of the second link plate, the hinge arm having a curved guide groove defined therein in which the hinge shaft is disposed.
 13. The drum washing machine according to claim 12, wherein the first link shaft is provided with a first torsion spring to elastically push the first link plate in a direction opposite to a direction in which the first link plate is rotated, and the second link shaft is provided with a second torsion spring to elastically push the second link plate in a direction identical to a direction in which the second link plate is rotated.
 14. The drum washing machine according to claim 12, wherein the support planes may each include a rib to prevent the hinge arm located on the support plane from being separated from the support plane.
 15. The drum washing machine according to claim 12, wherein each of the hinge arms is moved while the curved guide groove of the hinge arm is guided by the hinge shaft in the rotation direction to maintain a radius of rotation of the second door.
 16. The drum washing machine according to claim 1, wherein a center of the first door coincides with a center of the second door when the first door and the second door are simultaneously opened.
 17. A drum washing machine, comprising: a machine body having a first opening; a water tub disposed in the machine body, the water tub having a second opening facing the first opening; a rotary drum rotatably provided in the water tub to contain laundry; a first door rotatably provided at the machine body to open and close the first opening; and a second door rotatably provided at the tub to open and close the second opening, wherein a distance between a first rotational track formed by an end of the first door and a second rotational track formed by an end of the second door is uniformly maintained upon opening and closing of the first door and the second door.
 18. The drum washing machine according to claim 17, wherein the first door is provided with a coupling unit to couple the first door and the second door such that the second door is simultaneously opened when the first door is opened.
 19. The drum washing machine according to claim 18, wherein the coupling unit includes a hook unit configured to be hooked by and released from a hook coupling part provided at the second door.
 20. The drum washing machine according to claim 19, wherein the coupling unit further includes an actuator to actuate the hook unit to be released from the hook coupling part and a hook holding lever to hold the hook unit released by the actuator.
 21. The drum washing machine according to claim 20, wherein the actuator releases the hook unit during an operation of the drum washing machine. 